Wi-Fi Adaptive Receiver Diversity

ABSTRACT

Wireless communication devices with multiple receive (RX) chains may be operated to maintain high performance while saving power. This may be accomplished by evaluating signal strength during transmission of the RX packets, and/or evaluating a possible imbalance (gain difference) between the multiple RX chains within the wireless communication device. Signal strength (or good signal) detection may be enabled when non-MIMO (non-multiple-in-multiple-out) transmissions are taking place, while imbalance detection (antenna gain comparison) may be enabled when a specified number of single-stream packets have been received. Once the decision has been made to operate in a reduced number RX path mode, decision to reactivate one or more additional RX paths may be made based on MIMO detection, a detection of a drop in signal quality, and/or upon expiration of a power save timer.

PRIORITY CLAIM

This application claims benefit of priority of U.S. Provisional PatentApplication Ser. No. 62/074,599 titled “WiFi Adaptive Receiver Diversityand Transmit Antenna Selection”, filed on Nov. 3, 2014, which is herebyincorporated by reference as though fully and completely set forthherein.

FIELD OF THE INVENTION

The present application relates to wireless communications, and moreparticularly to techniques for adaptive receiver diversity in a wirelesscommunication device.

DESCRIPTION OF THE RELATED ART

Wireless communication systems are rapidly growing in usage. Further,wireless communication technology has evolved from voice-onlycommunications to also include the transmission of data, such asInternet and multimedia content. A popular short/intermediate rangewireless communication standard is wireless local area network (WLAN).Most modern WLANs are based on the IEEE 802.11 standard (or 802.11, forshort) and are marketed under the “Wi-Fi” brand name. WLAN networks linkone or more devices to a wireless access point, which in turn providesconnectivity to the wider area Internet.

In 802.11 systems, devices that wirelessly connect to each other arereferred to as “stations”. Wireless stations can be either wirelessaccess points (AP) or wireless clients (or client devices). Accesspoints (APs), which are also referred to as wireless routers, act asbase stations for the wireless network. APs transmit and receive radiofrequency signals for communication with wireless client devices, whichmay include a variety of different wireless communication devices,including portable devices, wearable devices, stationary devices and thelike. APs can also typically couple to the Internet in a wired fashion.As noted above, wireless clients or wireless client devices operating onan 802.11 network can be any of various devices such as laptops, tabletdevices, smart phones, or fixed devices such as desktop computers.Wireless client devices are also referred to herein as user equipment(or UE for short). Some wireless client devices are also collectivelyreferred to herein as mobile devices (although, as noted above, wirelessclient devices overall may be stationary devices as well).

In cellular and Wi-Fi systems, UEs sometimes have two receiver chainsand one or more transmit chains. The two receiver chains can be usedtogether to improve the receiver performance, but oftentimes at theexpense of using more power. In addition, UEs may also have two (ormore) antennas used for receiver diversity. However, furtherimprovements are needed to enable a UE to make better decisionsregarding use of single or multiple receiver chains and/or multipletransmitter chains, and use of single and/or multiple antennas.

SUMMARY OF THE INVENTION

Embodiments described herein relate to wireless communications, anddetermining whether to use one or more receiver chains and/ortransmitter chains in wireless communication systems, such as Wi-Fisystems.

In one set of embodiments, a UE includes multiple antennas, multipleradios, and one or more processors coupled to the radios. At least oneradio of the multiple radios performs Wi-Fi communications. The UE mayperform voice and/or data communications, as well as the methodsdescribed herein.

In some embodiments, UEs with multiple receive (RX) chains co-locatedwith one or more transmit (TX) chains may be operated to maintain highperformance while also saving power. This may be accomplished byevaluating signal strength during transmission of the RX packets, and/orevaluating a possible imbalance (gain difference) between the multipleRX chains within the UE. Signal strength (or good signal) detection maybe enabled when non-MIMO (non-multiple-input-multiple-output)transmissions are taking place, while imbalance detection (antenna gaincomparison) may be enabled when a specified number of single-streampackets have been received. Once the decision has been made to operatein a single (or more generally a reduced number) RX path mode, thedecision to reactivate or turn back on one or more additional RX pathsmay be made based on MIMO detection, a detection of a drop in signalquality, and/or upon expiration of a power save timer. In this context,reactivating an RX chain/path or turning an RX chain/path back on refersto using the RX path for receiving and processing a received signal.

This Summary is intended to provide a brief overview of some of thesubject matter described in this document. Accordingly, it will beappreciated that the above-described features are merely examples andshould not be construed to narrow the scope or spirit of the subjectmatter described herein in any way. Other features, aspects, andadvantages of the subject matter described herein will become apparentfrom the following Detailed Description, Figures, and Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present subject matter can be obtainedwhen the following detailed description of the embodiments is consideredin conjunction with the following drawings.

FIG. 1 illustrates an exemplary WLAN communication system, according tosome embodiments;

FIG. 2 illustrates a simplified block diagram of an exemplary WLANAccess Point (AP), according to some embodiments;

FIG. 3 illustrates a simplified block diagram of an exemplary mobiledevice (UE), according to some embodiments;

FIG. 4 illustrates an exemplary transceiver configuration that includesa control system for switching between antennas, according to someembodiments;

FIG. 5 illustrates an exemplary control system for switching between asingle/reduced number receive chain mode of operation and a multiplereceive chain mode of operation, according to some embodiments;

FIG. 6 illustrates an exemplary filter adaptation system for antennaimbalance evaluation, according to some embodiments;

FIG. 7 shows an exemplary diagram illustrating received signal strengthindication (RSSI) for two antennas, according to some embodiments;

FIG. 8 shows an exemplary diagram illustrating the cumulativedistribution function (CDF) for fading prediction error for an antennain a typical Wi-Fi channel, according to some embodiments;

FIG. 9 shows an exemplary diagram illustrating the CDF for fadingprediction error for an antenna in one of the worst case conditions in aWi-Fi channel, according to some embodiments;

FIG. 10 shows an exemplary control system for switching betweenantennas, according to some embodiments; and

FIG. 11 shows a flow diagram of an exemplary method for performingwireless communications during which one or more receive (RX) paths aredeactivated and reactivated, according to some embodiments.

While the features described herein are susceptible to variousmodifications and alternative forms, specific embodiments thereof areshown by way of example in the drawings and are herein described indetail. It should be understood, however, that the drawings and detaileddescription thereto are not intended to be limiting to the particularform disclosed, but on the contrary, the intention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the subject matter as defined by the appended claims.

DETAILED DESCRIPTION Acronyms

Various acronyms are used throughout the present application.Definitions of the most prominently used acronyms that may appearthroughout the present application are provided below:

UE: User Equipment

AP: Access Point

DL: Downlink (from BS to UE)

UL: Uplink (from UE to BS)

TX: Transmission/Transmit

RX: Reception/Receive

LAN: Local Area Network

WLAN: Wireless LAN

RAT: Radio Access Technology

TERMINOLOGY

The following is a glossary of terms used in this disclosure:

Memory Medium—Any of various types of non-transitory memory devices orstorage devices. The term “memory medium” is intended to include aninstallation medium, e.g., a CD-ROM, floppy disks, or tape device; acomputer system memory or random access memory such as DRAM, DDR RAM,SRAM, EDO RAM, Rambus RAM, etc.; a non-volatile memory such as a Flash,magnetic media, e.g., a hard drive, or optical storage; registers, orother similar types of memory elements, etc. The memory medium mayinclude other types of non-transitory memory as well or combinationsthereof. In addition, the memory medium may be located in a firstcomputer system in which the programs are executed, or may be located ina second different computer system which connects to the first computersystem over a network, such as the Internet. In the latter instance, thesecond computer system may provide program instructions to the firstcomputer for execution. The term “memory medium” may include two or morememory mediums which may reside in different locations, e.g., indifferent computer systems that are connected over a network. The memorymedium may store program instructions (e.g., embodied as computerprograms) that may be executed by one or more processors.

Carrier Medium—a memory medium as described above, as well as a physicaltransmission medium, such as a bus, network, and/or other physicaltransmission medium that conveys signals such as electrical,electromagnetic, or digital signals.

Computer System—any of various types of computing or processing systems,including a personal computer system (PC), mainframe computer system,workstation, network appliance, Internet appliance, personal digitalassistant (PDA), television system, grid computing system, or otherdevice or combinations of devices. In general, the term “computersystem” can be broadly defined to encompass any device (or combinationof devices) having at least one processor that executes instructionsfrom a memory medium.

User Equipment (UE) (or “UE Device”)—any of various types of computersystems devices which are mobile or portable and which performs wirelesscommunications using WLAN communication. Examples of mobile devicesinclude mobile telephones or smart phones (e.g., iPhone™, Android™-basedphones), and tablet computers such as iPad™, Samsung Galaxy™, etc.Various other types of devices would fall into this category if theyinclude Wi-Fi or both cellular and Wi-Fi communication capabilities,such as laptop computers (e.g., MacBook™), portable gaming devices(e.g., Nintendo DS™, PlayStation Portable™, Gameboy Advance™, iPhone™),portable Internet devices, and other handheld devices, as well aswearable devices such as smart watches, smart glasses, headphones,pendants, earpieces, etc. In general, the term “mobile device” can bebroadly defined to encompass any electronic, computing, and/ortelecommunications device (or combination of devices) which is easilytransported by a user and capable of wireless communication using WLAN.

Base Station (BS)—The term “Base Station” has the full breadth of itsordinary meaning, and at least includes a wireless communication stationinstalled at a fixed location and used to communicate as part of awireless telephone system or radio system.

Processing Element—refers to various elements or combinations ofelements that are capable of performing a function in a device, e.g. ina user equipment device or in a cellular network device. Processingelements may include, for example: processors and associated memory,portions or circuits of individual processor cores, entire processorcores, processor arrays, circuits such as an ASIC (Application SpecificIntegrated Circuit), programmable hardware elements such as a fieldprogrammable gate array (FPGA), as well any of various combinations ofthe above.

Wireless Device—any of various types of computer systems devices whichperforms wireless communications using WLAN communications. As usedherein, the term “wireless device” may refer to a UE device, as definedabove, or to a stationary device, such as a stationary wireless clientor a wireless base station. For example a wireless device may be anytype of wireless station of an 802.11 system, such as an access point(AP) or a client station (UE).

WLAN—The term “WLAN” has the full breadth of its ordinary meaning, andat least includes a wireless communication network or RAT that isserviced by WLAN access points and which provides connectivity throughthese access points to the Internet. Most modern WLANs are based on IEEE802.11 standards and are marketed under the name “Wi-Fi”. A WLAN networkis different from a cellular network.

Wi-Fi—The term “Wi-Fi” has the full breadth of its ordinary meaning, andat least includes a wireless communication network or radio accesstechnology (RAT) that is serviced by wireless LAN (WLAN) access pointsand which provides connectivity through these access points to theInternet. Most modern Wi-Fi networks (or WLAN networks) are based onIEEE 802.11 standards and are marketed under the name “Wi-Fi”. A Wi-Fi(WLAN) network is different from a cellular network.

Personal Area Network—The term “Personal Area Network” has the fullbreadth of its ordinary meaning, and at least includes any of varioustypes of computer networks used for data transmission among devices suchas computers, phones, tablets and input/output devices. Bluetooth is oneexample of a personal area network. A PAN is an example of a short rangewireless communication technology.

Automatically—refers to an action or operation performed by a computersystem (e.g., software executed by the computer system) or device (e.g.,circuitry, programmable hardware elements, ASICs, etc.), without userinput directly specifying or performing the action or operation. Thusthe term “automatically” is in contrast to an operation being manuallyperformed or specified by the user, where the user provides input todirectly perform the operation. An automatic procedure may be initiatedby input provided by the user, but the subsequent actions that areperformed “automatically” are not specified by the user, i.e., are notperformed “manually”, where the user specifies each action to perform.For example, a user filling out an electronic form by selecting eachfield and providing input specifying information (e.g., by typinginformation, selecting check boxes, radio selections, etc.) is fillingout the form manually, even though the computer system must update theform in response to the user actions. The form may be automaticallyfilled out by the computer system where the computer system (e.g.,software executing on the computer system) analyzes the fields of theform and fills in the form without any user input specifying the answersto the fields. As indicated above, the user may invoke the automaticfilling of the form, but is not involved in the actual filling of theform (e.g., the user is not manually specifying answers to fields butrather they are being automatically completed). The presentspecification provides various examples of operations beingautomatically performed in response to actions the user has taken.

Configured to—Various components may be described as “configured to”perform a task or tasks. In such contexts, “configured to” is a broadrecitation generally meaning “having structure that” performs the taskor tasks during operation. As such, the component can be configured toperform the task even when the component is not currently performingthat task (e.g., a set of electrical conductors may be configured toelectrically connect a module to another module, even when the twomodules are not connected). In some contexts, “configured to” may be abroad recitation of structure generally meaning “having circuitry that”performs the task or tasks during operation. As such, the component canbe configured to perform the task even when the component is notcurrently on. In general, the circuitry that forms the structurecorresponding to “configured to” may include hardware circuits.

Various components may be described as performing a task or tasks, forconvenience in the description. Such descriptions should be interpretedas including the phrase “configured to.” Reciting a component that isconfigured to perform one or more tasks is expressly intended not toinvoke 35 U.S.C. §112, paragraph six, interpretation for that component.

FIG. 1—Exemplary WLAN System

FIG. 1 illustrates one example of a WLAN system according to someembodiments. As shown, the exemplary WLAN system includes a wirelessclient station, or use equipment (UE) 106 communicating over a wirelesscommunication channel 142 with an Access Point (AP) 112. The AP 112 maycommunicate via a wired or wireless communication channel 150 with oneor more other electronic devices (not shown) and/or another network 152,such as the Internet. Additional electronic devices, such as the remotedevice 154, may communicate with components of the WLAN system via thenetwork 152. For example, the remote device 154 may be another wirelessclient station. The WLAN system may be configured to operate accordingto any of various communications standards, such as the various IEEE802.11 standards, for example.

FIG. 2—Block Diagram of an Exemplary Access Point

FIG. 2 shows a block diagram of an exemplary Access Point (AP) 112. Itis noted that the block diagram of AP 112 of FIG. 2 is merely oneexample of a possible system. As shown, the AP 112 may includeprocessor(s) 204 which may execute program instructions for the AP 112.The processor(s) 204 may also be coupled to memory management unit (MMU)240, which may be configured to receive addresses from the processor(s)204 and translate those addresses to locations in memory (e.g., memory260 and read only memory (ROM) 250) or to other circuits or devices.

The AP 112 may include at least one network port 270. The network port270 may be configured to couple to a wired network and provide aplurality of devices, such as mobile devices 106, access to theInternet. For example, the network port 270 (or an additional networkport) may be configured to couple to a local network, such as a homenetwork or an enterprise network. For example port 270 may be anEthernet port. The local network may provide connectivity to additionalnetworks, such as the Internet. The AP 112 may include at least oneantenna 234, which may operate as a wireless transceiver and maycommunicate with mobile device 106 via wireless communication circuitry(also referred to as radio) 230. AP 112 may use antenna 234 tocommunicate with the wireless communication circuitry 230 viacommunication chain 232. For example, AP 112 may use antenna 234 toreceive signals, and relay the received signals to radio 230 viacommunication chain 232. Similarly, AP 112 may use antenna 234 totransmit signals provided to antenna 230 from radio 230 viacommunication chain 232. Accordingly, communication chain 232 maycomprise one or more receive (RX) chains, one or more transmit (TX)chains or both. The wireless communication circuitry 230 may beconfigured to communicate via Wi-Fi or WLAN, e.g., 802.11. The wirelesscommunication circuitry 230 may also, or alternatively, be configured tocommunicate via various other wireless communication technologies,including, but not limited to, Long-Term Evolution (LTE), LTE Advanced(LTE-A), Global System for Mobile (GSM), Wideband Code Division MultipleAccess (WCDMA), CDMA2000, and the like. for example when the AP 112 isco-located with a base station in case of a small cell, or in otherinstances when it may be desirable for the AP 112 to communicate viavarious different wireless communication technologies.

The processor(s) 204 of the AP 112 may be configured to implement partor all of the methods described herein, e.g., by executing programinstructions stored on a memory medium (e.g., a non-transitorycomputer-readable memory medium). Alternatively, the processor 204 maybe configured as a programmable hardware element, such as an FPGA (FieldProgrammable Gate Array), or as an ASIC (Application Specific IntegratedCircuit), or a combination thereof. Furthermore, processor(s) 204 may bea processing element as described in the Terminology section above.

FIG. 3—Block Diagram of an exemplary Client Station

FIG. 3 illustrates the simplified block diagram of an exemplary UE 106,according to some embodiments. As shown in FIG. 3, the UE 106 mayinclude a system on chip (SOC) 300, which may include portions forvarious purposes. The SOC 300 may be coupled to various other circuitsof the UE 106. For example, the UE 106 may include various types ofmemory (e.g., including NAND flash 310), a connector interface 320(e.g., for coupling to a computer system, dock, charging station, etc.),the display 360, cellular communication circuitry 330 such as for LTE,GSM, etc., and short range wireless communication circuitry 329 (e.g.,BLUETOOTH™ and WLAN circuitry). The UE 106 may further include one ormore smart cards 310 that may have SIM (Subscriber Identity Module)functionality, such as one or more UICC(s) (Universal Integrated CircuitCard(s)) cards 310. The cellular communication circuitry 330 may coupleto one or more antennas, such as antennas 335 and 336 as shown. Theshort range wireless communication circuitry 329 may also couple to oneor more antennas, such as antennas 337 and 338 as shown. Alternatively,the short range wireless communication circuitry 329 may couple to theantennas 335 and 336 in addition to, or instead of, coupling to theantennas 337 and 338. The short range wireless communication circuitry329 may comprise multiple RX chains and/or multiple TX chains forreceiving and/or transmitting multiple spatial streams, such as in amultiple-input multiple output (MIMO) configuration.

As shown, the SOC 300 may include processor(s) 302 which may executeprogram instructions for the UE 106 and display circuitry 304 which mayperform graphics processing and provide display signals to the display360. The processor(s) 302 may also be coupled to memory management unit(MMU) 340, which may be configured to receive addresses from theprocessor(s) 302 and translate those addresses to locations in memory(e.g., memory 306, read only memory (ROM) 350, NAND flash memory 310)and/or to other circuits or devices, such as the display circuitry 304,cellular communication circuitry 330, short range wireless communicationcircuitry 329, connector interface (I/F) 320, and/or display 360. TheMMU 340 may be configured to perform memory protection and page tabletranslation or set up. In some embodiments, the MMU 340 may be includedas a portion of the processor(s) 302.

As noted above, the UE 106 may be configured to communicate wirelesslyusing one or more radio access technologies (RATs). The UE 106 may beconfigured to communicate according to a WLAN RAT for communication in aWLAN network, such as that shown in FIG. 1, for example. The UE 106 mayalso be configured to communicate on other RATs, such as cellular RATs,as desired.

As described herein, the UE 106 may include hardware and softwarecomponents for implementing the features described herein. For example,the processor(s) 302 of the UE 106 may be configured to implement partor all of the features described herein, e.g., by executing programinstructions stored on a memory medium (e.g., a non-transitorycomputer-readable memory medium). Alternatively (or in addition),processor 302 may be configured as a programmable hardware element, suchas an FPGA (Field Programmable Gate Array), or as an ASIC (ApplicationSpecific Integrated Circuit), or a combination thereof. In general,processor(s) 302 may be a processing element(s) as described in theTerminology section above. Alternatively (or in addition), processor(s)302 of the UE 106, in conjunction with one or more of the othercomponents 300, 304, 306, 310, 320, 330, 335, 340, 350, 360 may beconfigured to implement part or all of the features described herein.Accordingly, the UE 106 may implement switching between a “single RXchain mode of operation” and a “multiple RX chains mode of operation”,and may further implement switching between antennas, e.g. switchingbetween antennas 337 and 338. In some embodiments, short range wirelesscommunication circuitry 329 may include one or more TX chains and/or oneor more RX chains coupled to antennas 337 and 338.

As used herein, the term “UE” may refer to a device such as the UE 106described above.

Wi-Fi Adaptive Receiver Diversity FIG. 4: Example of Multiple ReceiverChains

FIG. 4 illustrates an exemplary UE with multiple receiver chains,including a first receiver chain referred to as RX0 and a secondreceiver chain referred to as RX1. The UE may also include at least onetransmitter chain referred to as TX0. As shown, RX0 and TX0 may sharethe same communication path 402, while RX1 may represent a singlecommunication path 404. As shown, RX0/TX0 402 is coupled to one input ofa Double Pole Double Throw (DPDT) switch 406. RX1 404 is coupled to theother input of the DPDT switch 406. A first output of the DPDT switch406 is coupled to antenna 0 (Ant 0), and a second output of the DPDTswitch is coupled to antenna 1 (Ant 1).

In one scenario, the two receiver chains RX0 and RX1 may be usedtogether (simultaneously) to improve the receiver performance, at theexpense of consuming more power. In another scenario, only one of thereceiver chains (e.g., either RX0 or RX1) may be turned on, and theother receiver chain (e.g., RX1 or RX0, respectively) may be turned off.This may occur, for example, when it is determined that the additionalreceive antenna would not help much in terms of reception ability, andthus one of the RX chains may be turned off to save power. In otherwords, under certain conditions it may be desirable to maintainperformance while also saving power, and it may not be necessary tosimultaneously operate the two RX chains to receive a beacon signal fromthe AP. While the simultaneous operation of the two antennas/receiversprovides the capability to extend the signal range, such may notnecessarily be a requirement. If the signal strength is good enough tosustain one RX chain (or a single RX chain) to receive a beacon signal,for example, then there may be no need to operate two (or more) RXchains simultaneously. For example, a legacy AP that doesn't supportMIMO (Multiple Input Multiple Output) mode may always transmit singlestream packets. When in close proximity to such an AP, signal strengthmay be good enough, therefore it may not be necessary or especiallyadvantageous to operate multiple RX chains and antennas. If one of theantennas becomes significantly compromised, for example by being inclose proximity to (or covered up) by a communications hand, then it maybe desirable to turn off (or disengage) that antenna. In the abovecases, one of the RX chains may be turned off without sacrificingperformance, and thereby save power. It should be noted that unlike forMIMO transmissions, it is not necessary to simultaneously operate two(or more) RX chains when receiving single stream packet transmissions.While in such cases the simultaneous operation of two (or more) RXchains may improve performance, such simultaneous operation is notnecessary. It should also be noted that while exemplary embodimentsdisclosed herein show two RX chains, alternate embodiments may includemore than two RX chains and more than two antennas, similarly configuredto be operated as described above.

When the UE device is in a power save state, i.e., when the UE deviceperiodically wakes up to receive a beacon from the AP (such as AP 112,for example) which is always transmitted in single stream, it may besufficient to operate one RX chain for beacon reception. When the UEdevice is in a continuous receive state, i.e., when the UE devicereceiver is always turned on (or engaged) in order to be prepared fordata transmission and/or reception, the opportunity for power savings isreduced, and both (or possibly more, in case of more than two) RX chainsare normally turned on. However, in a network that does not supportMIMO, (as mentioned above), when a signal is strong, one RX chain canachieve a peak receiver rate, and thus the second RX chain (or, ingeneral, any one or more additional RX chains) may be turned off. Asanother example, one antenna may be significantly compromised, e.g.,when a user's hand is gripping the UE casing in such a way as tocompromise or detract from the performance of one of the antennas. Insuch a case the RX chain corresponding to (or associated with) thecompromised antenna may be turned off.

FIG. 5: Determining Single or Multiple Receiver Chains

FIG. 5 shows a block diagram illustrating how to adaptively ordynamically determine when to utilize multiple receiver (RX) chains vs.a single RX chain, according to some embodiments. As shown in FIG. 5, onthe left hand side in the evaluation state 420, the UE is operating withmultiple RX chains. In one set of embodiments, there may be two RXchains. Alternatively or in addition, there may be more than two RXchains. The UE may enter the evaluation state 420 to evaluate whetheruse of one of the RX chains is to be discontinued, e.g., whether to dropdown to using only a single RX chain. In other words, while operating inthe evaluation state 420, both RX chains may be turned on, and the UEevaluates whether to turn one RX chain off, and if so, which RX chain.The evaluation may include imbalance detection 424, which may performantenna gain comparison and evaluation to determine if one of theantennas is operating in a compromised state, or in a lower gain statethan the other antenna. The evaluation may further include signalstrength detection, or good signal detection 422 as will be furtherdescribed below. As also shown in FIG. 5, the UE may enter theevaluation state 420 when either (or all) of a number of conditionsascertained during the course of the aforementioned evaluations aretrue.

As mentioned above, when operating in the evaluation state 420, the UEmay perform good signal detection, that is, it may track signal strength(such as a received signal strength indication, RSSI) and/or receivedsignal to noise ratio (SNR) for each RX chain separately, e.g. when notoperating in a MIMO network. In such cases, the UE may determine if itis receiving a good signal. Since during operation on a non-MIMO networkthe UE is not operating (not enabling or not engaging) both RX chains,the UE may perform a check to determine if the received signal strengthand/or received SNR are of at least some expected value(s). Responsiveto that determination, the UE may further determine whether one or moreof the RX chains may be disabled, e.g. switched off. The AP beacon cancarry all rates that are supported by the AP, hence the UE device isaware of whether MIMO operation is supported or not. When trackingreceived signal strength, the UE may only consider signals received froman associated AP (or the associated device with which the UE is incommunication). For the sake of simplicity and for the purposes ofillustration, in the example provided herein, communication of the UE iswith an associated AP. The UE thus monitors whether packets aresuccessfully received from that AP.

It should also be noted regarding single stream operation and MIMOoperation that either the AP has been explicitly instructed, or the UEhas been explicitly instructed that MIMO operation is not supportedand/or used. An implicit determination may be made from the AP adaptingto single stream from multiple streams due to certain conditions. Forexample, the observance of single stream packets from the AP over aperiod of time may be interpreted by a UE as lack of support for MIMO(whether long term or temporary), and the UE may therefore expect singlestream packets as opposed to multiple streams of packets in theforeseeable future.

If the UE device is in a power save state (receiving a beacon signalonly) or the network does not support MIMO, the RSSI/SNR of beaconframes transmitted by the associated AP may be used for filtering (todetermine whether to switch from multiple RX chain mode to single RXchain mode) as absolute signal strength may be needed to assess signalreception. For example, an IIR (infinite impulse response) filter may beused with filter coefficients adapted to the interval between twosuccessful beacon receptions. One RX chain may be switched off if theRSSI/SNR of any RX chain is larger than a specified threshold value. Forexample, the RX chain with the smaller antenna gain may be switched off,where the gain determination may be based on the results of theimbalance detection 424, as will be further discussed below. Thethreshold of the filter output may be related to the highest supportedrate and may also be adapted to the interval between successful beaconreceptions. For example, if the beacon rate with a single antenna is 1Mbps (megabits per second), then exemplary threshold values may bedefined as RSSI >−85 dBm and SNR >0 dB. At the highest 802.11n rate with1 antenna (MCS 7), exemplary threshold values may be defined asRSSI >−50 dBm and SNR >28 dB. In other words, for the filter (signalstrength evaluation as part of good signal detection 422) in power savemode, the threshold may be based on the beacon rate.

When the UE is in the evaluation state 420, it may perform imbalancedetection 424 upon having received N single stream packets. Overall, theimbalance detection 424 may be enabled with single stream transmission.This may occur when the network only supports single streamtransmission, or N previously received packets from the associated link(e.g. from an AP) are all single stream packets. The imbalance antennamay operate to gauge the antenna gain difference. In measuring theantenna gain difference, the measurements may be performed on allreceived packets, not just the beacon from the associated AP. In oneembodiment, the RSSI/SNR difference between different antennas may befiltered with an IIR filter with coefficients adapted to the intervalbetween two successfully received packets. The absolute value of thefilter output may be compared to a specified threshold value, and theantenna (and consequently also the corresponding RX chain) with thelower gain may be switched off or (temporarily) disabled or disengageduntil further action is taken to re-enable (engage) the antenna and thecorresponding RX chain. The specified threshold may be adapted to theinterval between two successfully received packets and may also beadapted to the RSSI/SNR of the packet destined for the device. Oneexample of a threshold value in this case is 15 dB.

If evaluation 420 results in determining either that there is goodsignal detection (on a non-MIMO network) or that there is detection ofan imbalance in N received single stream packets, then the UE maytransition to the single RX chain state (or mode of operation) 428,where only a single RX chain is used, or more generally where a reducednumber of RX chains are used.

When the UE transitions to the single RX (or more generally the reducednumber of RX) state 428, in which at least one RX chain is switched offas a result of evaluation 420 (indicated in FIG. 5 with T_(ev)=1), apower save timer (T_(ps)) may be started. The power save timer (T_(ps))may count a length of time (or time duration) during which power saveoperation is performed, i.e., a length of time during which (at least)one of the RX chains is turned off/disabled. Once the power save timer(T_(ps)) has expired, that is, once it has counted down to 0, it maytrigger the UE to leave the Single RX state 428 (in which only a singleRX chain is enabled) and transition back to the evaluation state 420(where multiple RX chains are enabled). Thus, in some embodiments, when(at least) one RX chain is switched off, a timer (power save timerT_(ps)) may be started. The duration of the timer may be any of variousvalues, e.g., 10 s. Upon timer expiration, both (or all) RX chains maybe turned on (engaged/enabled) for another step of evaluation 420.

The timer value or expiration could also be based on other information,e.g., motion of the UE. For example, when the UE is undergoing greatermotion, the length of the timer may be reduced so that the UE spendsless time in the single RX chain state. This may be beneficial as themotion of the UE introduces greater reception difficulties, and the UEwould therefore likely benefit from reducing the time period duringwhich it is operating with only a single enabled receiver. When the UEis undergoing less or no motion, the length of the timer may beincreased so that the UE operates in the single (reduced) RX chain state428 for a greater time duration (i.e. for a longer time period). Otherconditions that may cause the UE to leave the single RX chain state 428and transition back to the evaluation state 420 (i.e. multiple RX chainstate) may include detection of a signal drop or detection of MIMOoperations. In order to perform the signal drop detection, a good signalfilter may operate on the operating receiver chain to detect a signaldrop. If the signal filter output is lower than a specified thresholdvalue, then both (or more, if available and deemed beneficial to enable)RX chains may be turned on/enabled regardless of the timer value.Examples of these thresholds include RSSI <−60 dBm and/or SNR <20 dB.

That is, in Single RX chain mode 428, upon expiration of the power savetimer, both (or one or more additional) RX chains may be turned on inorder to perform the evaluation 420. That is, the presently operationalRX chain may be turned back on or enabled within a specified period oftime to again perform the evaluation on the two (or multiple) RX chains.Signal strength is monitored (434) in the single RX stage 428 toascertain whether signal reception is strong enough. For example, if alarge signal drop is observed (i.e. the signal is deteriorating) on thepresently operating RX chain, then the other antenna may be turned on tocompensate for the signal strength drop.

It should also be noted that upon transitioning to single RX chain mode(or state) 428, the AP may be explicitly instructed that presently onlysingle stream transmissions are being supported. However, such signalingmay be somewhat problematic, as it may need to be dissociated from thenetwork, then re-associated with the network, which may not be veryefficient. An alternative to such notification, following transition tothe single RX chain mode 428, may be to provide MIMO detection 436. Evenin single RX mode 428 the signal field of a MIMO transmission may bedecoded, since AP may indicate in the signal field when the packets aretransmitted in MIMO. Upon detecting such an indication of MIMOtransmissions, transition may be made back to dual RX mode 420. While aninitial packet may be lost in such cases, it does provide means tomaximize performance while saving power. To put it another way, whenimbalance detection 424 triggers a transition to a single (or reduced)RX state 428, the UE may either transmit explicit signaling to the AP toindicate that the UE now supports SISO (single input single output) onlytransmission. In case such signaling is implemented, MIMO detection 436may not be necessary. On the other hand, MIMO detection 436 may beperformed whereby a received HT (high throughput) signal field mayindicate a MIMO transmission.

When any one or more of the conditions (power save timer 432 elapses,signal drop detection 434 indicates dropped signal, MIMO detection 436indicates MIMO transmissions), the UE may transition from use of asingle RX chain (428) to using a plurality of RX chains (420). It shouldbe noted that imbalance detection 424 and MIMO detection 436 areapplicable in a continuous receive (RX) state.

FIG. 6—Filter Adaptation

FIG. 6 illustrates filter adaptation, according to some embodiments,whereby the imbalance between RX chains (or between respective antennasassociated with or corresponding to the RX chains) is determined. Asshown in FIG. 6, the RSSI/SNR of RX0 and the RSSI/SNR of RX1 may beinput to a node 440 whose output x(n)—representative of a differencebetween the RSSI/SNR of RX0 and the RSSI/SNR of RX1—is provided to afilter 442. The filter 442 outputs a value y(n) provided to a comparator446. The comparator 446 compares the value y(n) to a threshold value T.The result of this comparison may be used to determine if one of thereceiver chains (RX) may be turned on or off, that is, whether to enableor disable one (or more) of the RX chains.

One exemplary embodiment of filter 442, and the coefficients/values thatmay be implemented by the exemplary embodiment of filter 442 is providedbelow.

y(n)=(1−α(n))y(n−1)+α(n)x(n),  (1)

where x(n) and y(n) are the input and output, respectively, of filter442 at packet n. α(n) is the filter coefficient for packet n, anddepends on the inter-packet arrival time 444, that is, the time thatelapses between reception of packet n and reception of packet n−1 (thistime duration or time period is represented by “τn”).For example, considering y(1)=x(1), the following values may be used tokeep the filter time constant at a value of 1:

if τn<10 ms, α(n)= 1/128

if 10 ms<τn<20 ms, α(n)= 1/64

if 20 ms<τn<40 ms, α(n)= 1/32

if 40 ms<τn<80 ms, α(n)= 1/16

if 80 ms<τn<160 ms, α(n)=⅛

if 160 ms<τn<320 ms, α(n)=¼

if 320 ms<τn<1 s, α(n)=½

if 1 s<τn, α(n)=1.

Threshold T may also be adapted to τn, e.g., 10 dB for τn<100 ms, and 15dB for τn>100 ms. Further details regarding filter adaptation foradaptive TX antenna are detailed in the next section below.

It should also be noted that while exemplary embodiments herein includetwo RX chains, other embodiments may include additional RX chains andcorresponding (or associated) antennas, and any RX chain that remains inoperation may be considered an active RX chain while RX chains that havebeen deactivated may be considered inactive RX chains. Turning an RXchain on and turning an RX chain off may therefore also refer toactivating/enabling the RX chain and deactivating/disabling the RXchain, respectively.

FIG. 11—Exemplary Method for Enabling and Disabling RX Chains DuringWi-Fi Communications

FIG. 11 shows a flow diagram of an exemplary method for activating anddeactivating receive (RX) chains during wireless communications, forexample during Wi-Fi communications. As shown in FIG. 11, a wirelesscommunication device may operate using multiple active RX chains (1102).While operating using multiple active RX chains, the wirelesscommunication device may determine a signal strength of signals carryingRX packets received by the wireless communication device during non-MIMOtransmissions (1104). The wireless communication device may also detectan imbalance between respective antennas corresponding to a plurality ofRX chains in the wireless communication device upon receiving aspecified number of single-stream RX packets (1106). Based on thedetermination (1104) and detection (1106) the wireless communicationdevice may determine whether to deactivate one or more RX chains (1108).If the decision at 1108 is to deactivate one or more RX chains (“Yes”branch taken), the wireless communication device may deactivate one ormore RX chains and proceed to operate using a reduced number of activeRX chains (1110).

While operating using a reduced number of active RX chains, a timer maybe started (1112), and upon expiration of the timer (“Yes” branch at1114) the wireless communication device may return to operating usingmultiple active RX chains (1102). The wireless communication device mayalso operate to evaluate the signal strength of signals carrying packetsreceived at an active RX chain of the wireless communication device(1116). If the signal strength is not at a desired level (“No” branch at1120), the wireless communication device may return to operating usingmultiple active RX chains (1102). In addition, the wirelesscommunication device may also determine whether packets received at anactive RX chain of the wireless communication device were part of a MIMOtransmission (1118), and if the packets indicate a MIMO transmission(“Yes” branch at 1122), the wireless communication device may againreturn to operating using multiple active RX chains (1102).

Wi-Fi Adaptive TX Antenna Selection

Referring again to FIG. 4, the respective antenna gains of the twoantennas Ant0 and Ant1 may not be equal to each other, and thedifference between the respective gains may also change due to theenvironment, conductive materials coming into contact with the antennas,or due to a variety of other factors. At the receiver side, RF (radiofrequency) streams from both antennas may be used for receiverdiversity. At the transmitter side (TX side), in some cases it may bebeneficial to select the better antenna to obtain better performance,for example in embodiments that include only a single TX chain. Thus,for example in a Wi-Fi system having at least two RX chains and at leastone TX chain co-located with one of the RX chains, there is a choice ofwhich antenna to select for use with the TX chain to transmit. AdaptiveTX antenna selection as further described below may be performedindependently of whether adaptive receiver diversity as described aboveis being performed. In some embodiments, adaptive TX antenna selectionmay be used together with adaptive receiver diversity for furtherimproved performance and power savings. Alternatively, adaptive TXantenna selection may be used without adaptive receiver diversity,and/or adaptive receiver diversity may be used without adaptive TXantenna selection.

Antenna gain performance is typically not fixed or static. That is,antenna gain performance typically varies during operation. As mentionedabove, the gain performance may change as a result of a change inenvironmental factors (e.g. interference) or physical factors (e.g.conductive materials such as a hand coming into contact with theantenna), as well as some other factors. Furthermore, even if theantenna gain itself does not change, performance may be affected byother environmental factors, such as fading. All of the above may causean overall gain in one RX chain to differ from an overall gain in theother RX chain from time to time.

It may therefore be desirable to select the antenna which would providethe best performance for the TX chain (e.g. TX0 402 in FIG. 4). In someembodiments, selection of the antenna may be performed based on RXperformance. The two (or multiple) RX chains may be monitored toascertain performance when transmitting using a first antenna (e.g.antenna 337 in FIG. 3) vs. transmitting using a second antenna (e.g.antenna 338 in FIG. 3). Since Wi-Fi is a TDD (Time Division Duplex)system, it may take advantage of the reciprocity of cell conditionsbeing the same during TX and RX. That is, TX performance may bepredicted on a per-antenna basis, based on the individual RX performanceof both antennas.

FIG. 7—Short Term Fading and Antenna Gain

FIG. 7 illustrates short term fading and antenna gain associated withtwo RX chains, according to some embodiments. The RSSI is plotted on theY-axis with respect to elapsed time represented by the X-axis. It shouldbe noted that the instantaneous channel gain of RX0 (illustrated bycurve 450) is generally higher than that of RX1 (illustrated by curve454). However, at certain times the instantaneous channel gain of RX1may be higher than that of RX0, for example due to short term fading. Asillustrated in FIG. 7, in this instance the average gain of RX0(illustrated by line 452) is higher than the average gain of RX1(illustrated by line 456), with a gain difference of about 10 dB.Antenna gain difference may be measured by filtering out the short termfading. All received signals, Wi-Fi packets or other co-channelinterference may be used to measure relative antenna performance.Packets received from the AP may be used to measure the short termfading likely experienced by immediate transmissions.

FIG. 7 may be considered a snapshot of total-channel-class-antenna-gaintracking channel-gain-performance. There is an average 10 dB gaindifference, but instantaneous changes may be in opposite direction fromthe average trend, as shown in circled portion 458, for example. Thereciprocity between the TX channel and RX channel allows forconsideration of antenna performance for signal transmission tofollow/match antenna performance observed during signal reception. Forexample, certain antenna performance observed on the receive side(including the circled portion 458) may be expected to be the same orvery similar—within a specified time delay from having received an RXpacket—on the transmit side. If quick switching is possible, then it maybe possible to track the actual fading (which typically changes fairlyquickly). If, on the other hand quick switching is not possible, it maybe more desirable to choose the antenna with the higher averageperformance.

It may be ascertained whether fading may be followed (i.e. whether theswitching between antennas may be performed fast enough). FadingDoppler, or fading variation in Wi-Fi is typically slower than in manyother TDD systems. For example, when transmitting a packet/signal aspecified time period (e.g. 5 ms) after having received a packet/signal,channel conditions similar to those observed during the receive cyclemay also be observed during the transmit cycle. In FDD (frequencydivision duplex) systems, the RX chain cannot be used to predict thefading characteristics on the TX chain, whereas in certain TDD systems(such as Wi-Fi) such a prediction is possible.

FIGS. 8 and 9—Fading Prediction

FIG. 8 shows a diagram illustrating the cumulative distribution function(CDF) for fading prediction error for an antenna in a typical Wi-Fichannel (labeled ChB). As seen in FIG. 8, the CDF is represented on theY-axis, and is plotted versus the fading prediction error (in dB)represented on the X-axis. As previously mentioned, Wi-Fi is a TDDsystem where signal transmission and signal reception share the samefrequency band, and hence the UL (TX) and DL (RX) channels arereciprocal. Based on reciprocity, the UE may predict TX channel fadingbased on RX channel fading as long as transmission is within coherencetime of the fading channel. Coherence time represents the time durationover which the channel impulse response is considered to not be changingor varying. As seen in FIG. 8, when transmitting a packet 384 μs afterreceiving a packet, the difference between channel conditionsexperienced by the RX packet and the TX packet is fairly small, or atmost a specified value deemed acceptable. As the time delay between datareception and data transmission grows, the fading prediction error growsalong with it, making it more difficult to accurately predict thechannel conditions. In order to make use of the benefit that TDD allowsin making decisions regarding antenna switching for data packettransmission based on analysis that uses received data packets, packettransmission is expected to take place within a certain time window ofpacket reception. Thus, the TDD structure may be exploited withinspecific time windows.

FIG. 9 shows a diagram illustrating the CDF for fading prediction errorfor an antenna in one of the worst case conditions in a Wi-Fi channel.The Flat Rayleigh fading curves in FIG. 9 represent one of the worstcase conditions for fading error prediction (vs. ChB which is typicalWi-Fi channel model). As observed in FIG. 9, the fading prediction errorbegins deviating significantly as the time window between when packetsare received and when packets are transmitted takes on larger values(beginning at 1 ms) and increases. When it is not possible to transmitpackets within the required time window, a viable alternative decisionmaking process may be employed based on long term average antenna gain,as will be further described below.

Long Term Antenna Gain

In one set of embodiments, a filter may be utilized to filter out shortterm fading in order to evaluate the long term antenna gain difference.The long term antenna gain difference may be measured over a first timeinterval many times longer than a second time interval over which shortterm, e.g. instant variations in fading are measured. For example, insome embodiments, the first time interval may be tens, hundreds, or moretimes longer than the second time interval. In this manner the (longterm) imbalance between the respective gains of the different antennasmay be evaluated. Because the relative gain difference is evaluated, allreceived packets may be used for this evaluation. Overall, then, the RXchannel (and any antenna(s) associated with the RX channel) with betteraverage gain may be selected. While in cellular communications thesignal strength may be measured based on a regularly transmittedreference signal or periodic beacon/pilot signal, no such signal istransmitted in Wi-Fi. Consequently, for Wi-Fi communications signalstrength may be measured during packet transmissions. Measurements maybe made based on either the preamble or the packet. When exploitingreciprocity for instant fading, e.g. short term fadingdetection/prediction, RX packets used for (signal) measurements mayoriginate from the AP to which the packets are intended to betransmitted during TX cycles. In other words, the transmission of the RXpackets on which the signal measurements are based may be received fromthe AP with which the UE communicates.

When performing long term antenna gain detection/prediction, the sourceof the packets is of no importance, in contrast to the instant fadingcase. It doesn't matter where the packets are transmitted from, sincethey are all received through the same antenna, and therefore all willbe experiencing the same antenna gain. In other words, when performingthe evaluations for long term antenna gain, all received packets may beused. Which evaluation method to choose (i.e. instant fading or longterm antenna gain) may be determined based on at least the delay timer.As shown in FIG. 10 (and as will be further discussed in more detailbelow), the delay timer 508 may track the time that has elapsed sincethe last packet was received from the associated AP.

In one set of embodiments, an antenna switching control system may usean IIR filter as shown in FIG. 6 (and as also used for adaptive RXdiversity) with adaptive coefficients and an adaptive threshold asfollows:

y(n)=(1−α(n))y(n−1)+α(n)x(n),  (2)

where x(n) and y(n) are again the input and output, respectively, of thefilter 504 at packet n, and α(n)≦1 is the filter coefficient for packetn. Coefficient and threshold adaptation may be based on the intervalthat elapses between the two consecutive filter inputs. Coefficients andthreshold may both increase as the input sample interval increases.After a currently selected antenna has been switched to a differentantenna, y(n) may be set to −y(n−1).

FIG. 10—Exemplary Antenna Switching Control System

FIG. 10 illustrates an exemplary antenna switching control system,according to some embodiments. As shown in FIG. 10, the antennaswitching control system includes an instant fading detection/predictionstage 518 and a long term antenna gain detection/prediction stage 520.Which of the two prediction stages is to be used may be determinedduring the SIFS (short interframe space, i.e. the time period requiredfor the wireless interface to process a received frame and respond witha response frame) or during a time slot before the transmission. TheRSSI/SNR may be measured on the preamble of all packets, and input tonode 502, from where the difference values may be provided to all filterbranches (address filter 504 and IIR filter 512), as well as into sampleinterval detection 516. However, the instant fading detection stage 518may only use the measurements on packets from the associated AP toexploit the reciprocity for future transmission. At the switchingoccasion (which, as previously mentioned, may occur during the SIFS orduring a time slot before transmission), if the instant fading detectionis based on a packet received no later than a specified time period Tbefore the intended transmit time (as determined at 510), then theinformation (output) provided by the instant fading detection/predictionstage 518 may be used. Otherwise, the information (output) provided bythe long term antenna gain detection/prediction stage 520 may be used.

In one set of embodiments, the witching decision of the instant fadingdetection may be based on the threshold Th0=Prediction error margin+ΔANT(detected at 506). E.g., when the prediction error margin is 3 dB, thegain difference (ΔANT) between antenna 1 (ANT0) and antenna 2 (ANT1) maybe determined as follows:

ΔANT=ANT0 TX gain−ANT0 RX gain−ANT1 TX gain+ANT1 RX gain,  (3)

when ANT0 is connected to (is associated with) RX0, and ANT1 isconnected to (associated with) RX1.

Switching decision of the long term antenna gain detection may be basedon the threshold Th1=ΔTh+ΔANT where ΔTh is adapted to the sampleinterval (as detected at 514). It should be noted that as used in FIG.10, “switching decision” or “antenna switch decision” refers to whetheror not to switch between antennas. In other words, in the control systemof FIG. 10, two decisions are made. One is the decision of which output,the one from stage 518 or the one from stage 520 to use, and the otheris whether the selected output indicates that the antenna should beswitched.

It should be noted that while the fading conditions may be reciprocal,there may be some offset between the transmission performance and thereception performance of the antenna that can be accounted for perantenna. Furthermore, regarding the sample interval adjustment 516, onegoal may be to try and maintain a fixed IIR filter time constant.Because of the equally spaced beacon/pilot signals in cellularimplementations, it may be more feasible to maintain a fixed timeconstant for the IIR filter in cellular implementations. In contrast, inthe case of Wi-Fi communications, packets are received at varying times,and it may become more challenging to maintain a fixed time constant inthe filter. Thus, the weighting may be changed according to the timespacing between the various packets. For example, if the spacing betweenthe received packets is small, the weighting may be changedappropriately to help maintain a fixed time constant. Accordingly, adifferent threshold may also be used for determining when to switchantennas.

Embodiments of the present disclosure may be realized in any of variousforms. For example some embodiments may be realized as acomputer-implemented method, a computer-readable memory medium, or acomputer system. Other embodiments may be realized using one or morecustom-designed hardware devices such as ASICs. Other embodiments may berealized using one or more programmable hardware elements such as FPGAs.

In some embodiments, a non-transitory computer-readable memory mediummay be configured so that it stores program instructions and/or data,where the program instructions, if executed by a computer system, causethe computer system to perform a method, e.g., any of a methodembodiments described herein, or, any combination of the methodembodiments described herein, or, any subset of any of the methodembodiments described herein, or, any combination of such subsets.

In some embodiments, a wireless device may be configured to include aprocessor (or a set of processors) and a memory medium, where the memorymedium stores program instructions, where the processor is configured toread and execute the program instructions from the memory medium, wherethe program instructions are executable to cause the wireless device toimplement any of the various method embodiments described herein (or,any combination of the method embodiments described herein, or, anysubset of any of the method embodiments described herein, or, anycombination of such subsets). The device may be realized in any ofvarious forms.

Although the embodiments above have been described in considerabledetail, numerous variations and modifications will become apparent tothose skilled in the art once the above disclosure is fully appreciated.It is intended that the following claims be interpreted to embrace allsuch variations and modifications.

1. A wireless communication device comprising: a plurality of antennasconfigured to transmit and receive signals; a plurality of receive (RX)chains, each RX chain corresponding to a respective antenna of theplurality of antennas and configured to receive signals from therespective antenna; and a processing element configured to interoperatewith the plurality of RX chains to cause the wireless communicationdevice to: determine a signal strength of signals carrying RX packetsreceived during non-MIMO (non-multiple-input-multiple-output)transmissions; detect an imbalance between respective antennas of theplurality of antennas upon receiving a specified number of single-streamRX packets; and determine whether or not to deactivate one or more RXchains of the plurality of RX chains responsive to at least one of thefollowing: results of the signal strength determination; and results ofthe imbalance detection.
 2. The wireless communication device of claim1, wherein the processing element is configured to interoperate with theplurality of RX chains to further cause the wireless communicationdevice to: start a timer when deactivating an RX chain of the pluralityof RX chains; and reactivate the RX chain upon expiration of the timer.3. The wireless communication device of claim 1, wherein the processingelement is configured to interoperate with the plurality of RX chains tofurther cause the wireless communication device to: evaluate a signalstrength of signals carrying packets received at an active RX chain ofthe plurality of RX chains; and reactivate one or more deactivated RXchains of the plurality of RX chains responsive to results of theevaluation of the signal strength.
 4. The wireless communication deviceof claim 1, wherein the processing element is configured to interoperatewith the plurality of RX chains to further cause the wirelesscommunication device to: detect when packets received at an active RXchain of the plurality of RX chains are part of a MIMO transmission; andreactivate one or more deactivated RX chains of the plurality of RXchains responsive to results of the detection of the MIMO transmission.5. The wireless communication device of claim 1, wherein the processingelement is configured to interoperate with the plurality of RX chains tofurther cause the wireless communication device to: base thedetermination of the signal strength of the signals carrying RX packetsreceived by the wireless device during non-MIMO transmissions on packetsreceived from an associated access point device to which the wirelessdevice also transmits packets.
 6. The wireless communication device ofclaim 1, wherein the processing element is configured to interoperatewith the plurality of RX chains to further cause the wirelesscommunication device to: perform the imbalance detection using aninfinite impulse response (IIR) filter.
 7. The wireless communicationdevice of claim 6, wherein the processing element is configured tointeroperate with the plurality of RX chains to further cause thewireless communication device to: keep a filter time coefficient of theIIR filter constant.
 8. The wireless communication device of claim 1,wherein the processing element is configured to interoperate with theplurality of RX chains to further cause the wireless communicationdevice to: upon transitioning to a state in which a single RX chain ofthe plurality of RX chains remains active, transmit explicit signalingto an access point device from which the wireless communication devicehas been receiving RX packets; wherein the explicit signaling indicatesto the access point device that the wireless communication device onlysupports single-input single-output transmissions.
 9. An apparatuscomprising: a processing element configured to: determine a signalstrength of signals carrying RX packets received during non-MIMO(non-multiple-input-multiple-output) transmissions; detect an imbalancebetween respective antennas of a plurality of antennas upon receiving aspecified number of single-stream RX packets; and determine whether ornot to deactivate one or more RX chains of a plurality of RX chainsresponsive to at least one of the following: results of the signalstrength determination; and results of the imbalance detection.
 10. Theapparatus of claim 9, wherein the processing element is furtherconfigured to: start a timer when deactivating an RX chain of theplurality of RX chains; and reactivate the RX chain upon expiration ofthe timer.
 11. The apparatus of claim 9, wherein the processing elementis further configured to: evaluate a signal strength of signals carryingpackets received at an active RX chain of the plurality of RX chains;and reactivate one or more deactivated RX chains of the plurality of RXchains responsive to results of the evaluation of the signal strength.12. The apparatus of claim 9, wherein the processing element is furtherconfigured to: detect when packets received at an active RX chain of theplurality of RX chains are part of a MIMO transmission; and reactivateone or more deactivated RX chains of the plurality of RX chainsresponsive to results of the detection of the MIMO transmission.
 13. Theapparatus of claim 9, wherein the processing element is furtherconfigured to: base the determination of the signal strength of thesignals carrying RX packets received during non-MIMO transmissions onpackets received from an associated access point device to which packetsare transmitted.
 14. The apparatus of claim 9, wherein the processingelement is further configured to: perform the imbalance detection usingan infinite impulse response (IIR) filter; and keep a filter timecoefficient of the IIR filter constant.
 15. The apparatus of claim 9,wherein the processing element is further configured to: when a singleRX chain of the plurality of RX chains remains active, cause explicitsignaling to be transmitted to an access point device from which RXpackets are received; wherein the explicit signaling indicates to theaccess point device that only single-input single-output transmissionsare supported.
 16. A non-volatile memory device storing instructionsexecutable by a processing element to cause a wireless communicationdevice to: determine a signal strength of signals carrying RX packetsreceived by the wireless communication device during non-MIMO(non-multiple-input-multiple-output) transmissions; detect an imbalancebetween respective antennas of a plurality of antennas within thewireless communication device upon the wireless communication devicereceiving a specified number of single-stream RX packets; and determinewhether or not to deactivate one or more RX chains of a plurality of RXchains within the wireless communication device responsive to at leastone of the following: results of the signal strength determination; andresults of the imbalance detection.
 17. The non-volatile memory deviceof claim 16, wherein the instructions are executable by the processingelement to further cause the wireless communication device to: start atimer when deactivating an RX chain of the plurality of RX chains; andreactivate the RX chain upon expiration of the timer.
 18. Thenon-volatile memory device of claim 16, wherein the instructions areexecutable by the processing element to further cause the wirelesscommunication device to: evaluate a signal strength of signals carryingpackets received at an active RX chain of the plurality of RX chains;and reactivate one or more deactivated RX chains of the plurality of RXchains responsive to results of the evaluation of the signal strength.19. The non-volatile memory device of claim 16, wherein the instructionsare executable by the processing element to further cause the wirelesscommunication device to: detect when packets received at an active RXchain of the plurality of RX chains are part of a MIMO transmission; andreactivate one or more deactivated RX chains of the plurality of RXchains responsive to results of the detection of the MIMO transmission.20. The non-volatile memory device of claim 16, wherein the instructionsare executable by the processing element to further cause the wirelesscommunication device to: when a single RX chain of the plurality of RXchains remains active, cause explicit signaling to be transmitted to anaccess point device from which the wireless communication devicereceives RX packets; wherein the explicit signaling indicates to theaccess point device that only single-input single-output transmissionsare supported by the wireless communication device.